A blue-color light-emitting element or ultraviolet light-emitting element can be used as a white light source if combined with an appropriate wavelength conversion material. Active studies have been conducted on applications of such white light sources to backlights for liquid crystal displays and the like, light-emitting diode illumination, automotive lighting, general lighting to replace fluorescent lighting, and so on. Some of the studies have already been put to practical use. Today, such blue-color light-emitting elements and ultraviolet light-emitting elements are produced mainly by growing a thin film of gallium nitride-based semiconductor crystal using a technique such as a metal-organic chemical vapor deposition process (MOCVD process) or molecular beam epitaxy process (MBE process) and are collectively referred to as gallium nitride-based light-emitting diodes or GaN-based LEDs.
Conventionally, most of the substrates used for GaN-based LEDs are sapphire substrates. Since sapphire and GaN differ greatly in lattice constant, a considerable dislocation on the order of 109/cm2 is unavoidable for a GaN crystal epitaxially grown on a sapphire substrate. However, sapphire substrates are more inexpensive than SiC substrates and GaN substrates. Moreover, since light-emitting efficiencies of InGaN in blue-color light-emitting regions normally used as quantum well layers of GaN-based LEDs are insufficiently sensitive to dislocation density, sapphire substrates are still used widely as primary substrates under the present circumstances.
However, when gallium nitride-based semiconductor crystals are viewed as a material for devices used under conditions of high carrier concentration, high dislocation density such as described above considerably deteriorates device characteristics. For example, high dislocation density remarkably reduces the life of devices such as high-power LEDs or lasers. Also, when an active layer structure contains no In such as when an AlGaN layer is used as an active layer, or when an active layer structure includes an InGaN layer or InAlGaN layer with a relatively small In content (e.g., about 0.1 or below) to realize light emission with a wavelength shorter than somewhere around, the near-ultraviolet region, the dependence of internal quantum efficiency on the dislocation density increases, and consequently luminous intensity itself decreases if the dislocation density is high unlike when an InGaN layer with a blue or longer emission wavelength is contained in the active layer structure. For these cases, it is useful to adopt a GaN substrate as a substrate for epitaxial growth. It is expected that this will reduce the dislocation density found in epitaxial layers to 108/cm2 or below, or even 107/cm2 or below. Furthermore, if dislocations and the like on the substrate and the like are reduced, the dislocation density is expected to be reduced to even 106/cm2 or below. That is, the dislocation density is expected to be reduced by two to three orders of magnitude or more compared to when a sapphire substrate is used. In view of these circumstances, freestanding GaN substrates and freestanding AlN substrates are suitable as substrates for epitaxial growth of gallium nitride-based semiconductor crystals.
As attempts to epitaxially grow gallium nitride-based semiconductor crystals on GaN substrates which are nitride substrates, examples on such attempts include Patent Document 1 (Japanese Patent Laid-Open No. 2005-347494), Patent Document 2 (Japanese Patent Laid-Open No. 2005-311072, and Patent Document 3 (Japanese Patent Laid-Open No. 2007-67454).
Patent Document 1 discloses a technique which involves using a nitride substrate ((0001)-plane GaN substrate) to epitaxially grow a GaN layer, cleaning the GaN substrate with a reactor pressure set to 30 kilopascals, growing a 1-μm-thick first n-type GaN buffer layer with the substrate temperature maintained at 1050° C. and the reactor pressure maintained at 30 kilopascals, subsequently stopping raw material supply once, and further forming a 1-μm-thick second n-type GaN buffer layer by heating the substrate to a substrate temperature of 1100° C. with the reactor pressure maintained at 30 kilopascals. This crystal growth method allegedly provides a semiconductor apparatus having buffer layers with excellent surface flatness and good crystal quality.
Patent Document 2 discloses an invention of a light-emitting element produced by removing organic and other contaminations and moisture adhered to the surface of a GaN substrate while improving surface crystallinity of the substrate by flows of hydrogen gas, nitrogen gas, and ammonia gas, forming a multilayer structure intermediate layer made up of GaN and InGaN layers on the GaN substrate using flows of nitrogen gas and hydrogen gas, and then forming a reflective layer, active layer, and gallium nitride-based semiconductor layer on the intermediate layer.
Example 26 in Patent Document 3 discloses an invention of a laser element produced by forming a 3-μm-thick n-type GaN buffer layer doped with Si on a GaN substrate and building a stacked structure on the n-type GaN buffer layer. Incidentally, the Patent Document states that a 300-Angstrom or thinner buffer layer formed at a low temperature of around 500° C. may be provided between the GaN buffer layer and GaN substrate.    Patent Document 1: Japanese Patent Laid-Open No. 2005-347494    Patent Document 2: Japanese Patent Laid-Open No. 2005-311072    Patent Document 3: Japanese Patent Laid-Open No. 2007-67454